Multi-nozzle tubular plasma deposition burner for producing preforms as semi-finished products for optical fibers

ABSTRACT

The invention relates to a multi-nozzle, tubular plasma deposition burner ( 1 ) for producing preforms as semi-finished products for optical fibers, wherein a media stream containing glass starting material and a carrier gas is fed to the burner ( 1 ), means for feeding at least one dopant using at least one precursor gas and a substantially perpendicular orientation of the burner gas longitudinal axis relative to the center axis of the substrate ( 4 ). According to the invention, a first partial stream of a first gas or gas mixture, in particular a precursor gas, is fed to the plasma and to the substrate ( 4 ) by way of at least one nozzle running in the burner longitudinal axis and a second partial stream of the first gas or of another gas or gas mixture, in particular a precursor gas, is fed to the plasma and the substrate by way of another nozzle ( 5 ); said gases or gas mixtures are fed in such a way that said partial streams combine in the vicinity of the substrate.

The invention relates to a multi-nozzle, tubular plasma deposition burner for producing preforms as semi-finished products for optical fibers, wherein a media stream containing glass starting material and a carrier gas is fed to the burner, means for feeding at least one dopant using at least one precursor gas and an essentially perpendicular orientation of the longitudinal axis of the burner relative to the center axis of the substrate according to the preamble of patent claim 1 as well as a method for producing preforms as semi-finished products for optical fibers.

Induction plasma burners have been known since the early 1960s and are preferably used to process materials, for the synthesis of fine-granular dusts and in spectroscopy as a stimulant source. Burners for the generation of inductive coupled plasmas at atmospheric pressure generally consist of three concentrically arranged quartz tubes. In known designs, the carrier gas is moving through a discharge tube with a small diameter under laminar flow. If the diameters of the discharge tube are larger, the carrier gas is moving in a whirl-like flow.

The use of the plasma method for the production of quartz glass with a low OH content and for the production of doped quartz glass as well as the utilization of the plasma method for the production of preforms in the manufacture of optical waveguides is known from DE 25 36 457. Various conditions must be met for the use to produce optical waveguides. For example, the core and cladding glass must consist of high-purity quartz glass and the cladding glass must have a defined index of refraction which is below the refraction number of the core glass to create a corresponding transmission.

To achieve a predefined reduction of the index of refraction of the cladding glass, the flame of the plasma burner is additionally fed with a hydrogen-rich fluorine compound that breaks down under heat exposure and is provided in vapor state.

The production of a rotation-symmetrical semi-finished product for the optical waveguide manufacture is successful in that a quartz cylinder is used as target or substrate and the deposition of defined fluorine-containing cladding glass layers onto the rotating core glass cylinder is performed during the simultaneous relative motion of the plasma burner to the core glass cylinder.

The previously mentioned DE 25 36 457 describes a burner consisting of three concentrically arranged quartz tubes. The outer tube is supposed to protrude from the middle and the latter the inner tube. The work gas and the silicon compound including the fluorine compound in vapor state are fed through the inner tube. Oxygen is introduced as a separating gas through the gap between the inner and the middle tube as well as between the middle tube and the outer tube. Flushing the middle and outer tube each with a separating gas has the advantage that no silicon oxide is deposited on the burner. The three quartz tubes are sealed against each other and against the exterior atmosphere in a metal frame. The actual induction coil is arranged around the free end of the outer tube and is fed by an HF generator. The work gas and the two separating gases are supplied by way of tangentially arranged lines. The plasma flame is ignited with argon gas. The SiCl₄ is decomposed by the high temperature of the plasma flame and reacts with oxygen to become SiO₂, which is deposited on the target and is simultaneously sintered. The fluorine donor is also decomposed by the high temperature of the plasma flame and fluorine is integrated into the deposited hyaline SiO₂.

Alternative plasma burner designs are disclosed, e.g., in DE 298 23 926 A1. In the plasma burner described there. undoped quartz glass is also preferably deposited on a rotating cylindrical symmetrical target. However, the goal there is to manufacture tubes and semi-finished products for the optical wave guide production rather than fluorine doped step-index preforms. The plasma gasses used in the mentioned prior art comprise nitrogen and oxygen in a suitable ratio. Media gases are SiCl₄, oxygen and a gaseous fluorine donor, preferably SF₆. The plasma burner itself consists of a tube-shaped burner case made of quartz glass. A copper induction coil is provided around the upper section of the outer quartz tube. Furthermore, a nozzle pair arranged on the side is provided, which is used to inject the precursors into the plasma. These injection nozzles consist of quartz glass and feature a small interior diameter as well as a special cross-section of the exit area. The injection openings are arranged diametrical and offset to each other.

DE 102 31 037 C1 relates to a method and a system for the production of a preform made of synthetic quartz glass by means of plasma-supported deposition methods.

A multi-nozzle deposition burner is used there, which is fed with a hydrogen-rich media flow comprising a starting glass material and a carrier gas. The starting glass material is fed into a plasma zone by means of the deposition burner and oxidized there under the formation of SiO₂ particles. The SiO₂ particles are deposited on a deposition area and vitrified directly in the process.

To increase the deposition efficiency, it is proposed there that the media flow is focused in the direction of the plasma zone by means of the deposition burner.

Based on the facts mentioned above, the object of the invention is therefore to provide a further developed multi-nozzle tubular plasma deposition burner for the production of preforms as semi-finished products for optical fibers which allows the improvement of the degree of the SiO₂ deposition on the substrate or target and the fluorine integration efficiency. For this purpose, a sub-object of the invention is to set defined deposition conditions by means of the deposition burner to be provided, which ensure the desired high deposition rate and reasonable layer quality. The ultimate goal of this is a reduction of the specific material and energy consumption and cost savings.

The object of the invention is solved on the appliance side by the combination of characteristics according to patent claim 1 as well as a method based on the teaching according to patent claim 23.

Accordingly, the basis is a multi-nozzle tubular plasma deposition burner for producing preforms as semi-finished products for optical fibers, wherein a media stream containing a glass starting material and a carrier gas is fed to the burner. Furthermore, means for feeding at least one dopant using at least one precursor gas are provided, wherein the longitudinal axis of the burner is essentially perpendicular relative to the center axis of the substrate.

According to the invention, a first partial stream of a first gas or gas mixture, in particular a precursor gas, is fed to the plasma and to the substrate by way of at least one nozzle running in the longitudinal axis of the burner and a second partial stream of the first gas or gas mixture or of another, different precursor gas, is fed to the plasma and to the substrate by way of another nozzle, in particular a nozzle pair in such a way that the partial streams merge in the vicinity of the substrate.

Accordingly, the precursor gas is initially fed almost perpendicular from below to the axis of the substrate by a tubular nozzle assembly, comprising, e.g., one or several circular, oval or square or polygonal cross sections as an outlet opening, wherein the cross section or the sum of cross sections is adjusted to the desired output volume.

According to the object, the degree of deposition is increased significantly by feeding an additional partial stream of the precursor gas through at least two nozzles, in particular nozzle chains, located on the side below the substrate close to the burner. In so doing, the nozzles or nozzle chains are arranged on the side in such a way that the precursor gas streams converge underneath the substrate and the streams in the center of the plasma stream are almost neutralized. The partial stream ratio can be set according to the substrate or target size.

In one embodiment, the at least one additional nozzle or the nozzle pair for the second partial stream is arranged offset to the longitudinal axis of the burner, wherein the nozzle pair in turn is provided at an angle deviating from the longitudinal axis of the burner. This angle can vary within a range of e.g., 45° to 135° relative to the longitudinal axis of the burner.

As mentioned above, the nozzle pair can comprise a nozzle chain, wherein nozzle chains of the nozzle pairs are facing each other at a defined angle, but usually not directly. In one embodiment, said angle mentioned above is adjustable.

The nozzle pair to create the second partial stream is provided outside the actual burner, namely away from the closest burner tube relative to the substrate in the space between the substrate surface and the burner.

According to the invention, more than two nozzle chains are provided, which create a tangential gas stream component relative to the tube configuration of the burner.

In one embodiment, the nozzle pair and the respective nozzle chains are designed mobile and adjustable, wherein the optimal position can optionally be adjusted automatically or manually during the coating process, depending on a number of process parameters such as, e.g., the moving direction of the substrate.

The burner comprises known tubes running in the longitudinal axis of the burner, wherein said tubes are designed as two interlocked tubes according to the invention. Said tubes can be made of a glass and/or ceramic material or of an organic polymer or a polymer composite. The use of a glass or ceramic material coated with an organic material, e.g., an organic foil is another option. At least one of the interlocked tubes is designed with thick walls, i.e., a thickness of, e.g., ≧2 mm. The wall thickness of the thickest tube can be selected in the range between ≧2 mm to essentially 20 mm.

The tubes are positioned in each other concentrically or deviating from the concentric arrangement, wherein the wall thickness ranges between >5 mm to essentially 20 mm.

The inside of the tube assembly has a greater distance from the plasma space than the outer tube of the tube assembly.

Furthermore, based on the invention it is provided that the precursor gas is fed through a chamber. i.e., the deposition burner has at least one of said chambers used to mix the gas or clam the gas stream.

The one or multi-link nozzle chains can be provided with a plate form with a plurality of identical or different nozzle drill holes. The nozzle drill holes can preferably be provided at an angle of 35° to 55°, meaning that this arrangement forces a helix stream.

In addition, it is possible to surround the burner tube assembly with a nitrogen curtain which again can be fed with a tube extending around it.

Moreover, the nozzle for the first partial gas stream running along the longitudinal axis of the burner can also comprise a nozzle group. As mentioned at the beginning, said nozzle group can comprise individual nozzles with different or adjustable nozzle cross sections or cross-sectional areas.

The greater wall thickness of the used tubes according to the invention, especially for the inner plasma burner tube, prevents the premature deposition of raw materials on the burner and increases the process stability and the service life of the burners.

In a preferred embodiment, the distance of the nozzles of the nozzle pair for the creation of the second partial stream ranges from 0.1 mm to 10 mm relative to the upper edge of the highest towering tube, i.e., the tube closest to the substrate.

Another embodiment of the invention provides the possibility to deliberately enlarge the distance of the nozzles of the nozzle pair to create the second partial stream relative to the upper edge of the highest towering plasma tube. Advantages resulting herefrom are generated by the then reduced temperature at the site of attachment or arrangement of the nozzles of the nozzle pair, so that temperature-related negative impacts on the fed gas or gas mixture can be prevented.

The nozzles for the creation of the first partial stream are designed in such a way that makes it possible to feed the gas at an angle deviating from the perpendicular relative to the longitudinal axis of the substrate. The feed angle is preferably defined within the range of between 35° and 55°, to allow the optimal routing of the glass soot onto the surface of the substrate in the high-rate deposition process while at the same time preventing the caking onto the burner tubes.

The combination of the deposition burner according to the invention with one or more gas burners results in a better clear fusibility in the high-rate deposition process, wherein the residual stress in the finished coated preform by means of the at least one additional burner is reduced toward zero. The use of said common gas burners is possible in particular if it concerns the manufacture of preforms whose fibers are used in the ultraviolet range, wherein high OH-contents are permitted.

The use of the deposition burner according to the invention and a respective method to be conducted makes it possible to use reaction mixtures of different precursor compounds. Therefore, it is easily possible to integrate several dopants into the glass matrix. The creation of partial gas streams makes it possible to use several silicon compounds at variable stoichiometric ratios and to add more additives.

Feeding at least two different precursor compounds by way of two separate nozzles makes it possible to feed even precursor compounds that react with each other by way of separate line paths. For instance, the first partial stream can be characterized by the SiCl₄ feed from the bottom side of the burner. Consequently, finely divided soot is formed from SiCl₄ and O₂ that reacts quickly with suitable F-donors due to its raised surface. Said F-donors can be fed by way of the second partial stream.

Because the nozzles of the nozzle pair for the generation of the second partial stream can be positioned individually and separately facing in a x, y and/or z direction, it is possible to improve the deposition rates overall, because the respective translational and rotational speed of the substrate/rod can be considered.

The inclination of the partial gas streams relative to the longitudinal axis of the burner makes it possible to optimally integrate the partial gas streams into the plasma volume stream.

Furthermore, it is possible to provide an additional nozzle chain outside the plasma burner to cool the rod and the burner, so that a defined local atmosphere surrounding the reaction space can be generated. If UV-absorbing materials are used, it is possible to reduce the ultraviolet exposure of the substrate while it is manufactured.

The invention is explained in detail below based on an exemplary embodiment as well as by means of figures.

FIG. 1 shows a basic cross-sectional illustration of the deposition burner according to the invention with its essential components and their arrangement with respect to a substrate, in particular a rotating core glass rod.

Herein the burner 1 comprises an outer burner tube 3 and an HF coil 6. The outer burner tube 3 essentially contains a first inner burner tube 2 with thick walls arranged concentrically relative to the former. This heat-resistant tube consists, e.g., of ceramics or glass and has a wall thickness of >5 mm to, e.g., 20 mm.

The greater wall thickness compared to the known models prevents the premature deposition of raw materials on the burner, thus increasing the process stability and the service life of the entire assembly.

A first partial stream of a precursor gas to be fed is fed to the substrate 4 from below, so to speak, through a tube 1 arranged underneath the tube 2. At least two nozzle chains 5 arranged offset opposite from each other generate the second tangentially oriented partial stream of the precursor gas. The figure herein only shows one of the nozzle chains 5.

Said nozzle pair 5 is arranged in such a way that the precursor gas streams converge below the substrate 4 and the streams in the center of the plasma neutralize each other. The nozzles 5 are provided at a distance A from the upper border of the outer tube 3 of the burner assembly.

Said distance can be in the range between 1 mm and 10 mm. The clearance angle between the nozzles 5 can be provided in the range between 45° and 90° wherein it is possible to design the angular position variably.

FIG. 2 shows a top view of the burner assembly according to the invention with a visible outer burner tube 3 as well as nozzle chains arranged offset opposite of each other and a section of the substrate 4, i.e., the core glass rod rotating during the coating process. The possible motion of the nozzle chains 5 in a longitudinal direction of the substrate 4 is symbolized by the arrows, wherein it is also possible to aim the nozzles or nozzle chains 5 alternatively or additionally out of or into the image level. 

1. Multi-nozzle, tubular plasma deposition burner for producing preforms as semi-finished products for optical fibers, wherein a media stream containing glass starting material and a carrier gas is fed to the burner, means for feeding at least one dopant using at least one precursor gas and a substantially perpendicular orientation of the longitudinal axis of the burner relative to the center axis of the substrate, wherein a first partial stream of a first gas or gas mixture, in particular a precursor gas, is fed to the plasma and to the substrate by way of at least one nozzle running in the longitudinal axis of the burner and a second partial stream of the first or of another gas or gas mixture, in particular a precursor gas, is fed to the plasma and to the substrate by way of another nozzle in such a way that said partial streams merge in the vicinity of the substrate.
 2. Burner according to claim 1, wherein the other nozzle is provided as multi-nozzle or nozzle pair, comprising two or more individual nozzles arranged offset relative to the longitudinal axis of the burner.
 3. Burner according to claim 1, wherein the other nozzle or the nozzles of the nozzle pair are arranged at an angle deviating from the longitudinal axis of the burner.
 4. Burner according to claim 2, wherein the nozzle pair comprises a group of individual nozzles as a nozzle chain, wherein the nozzle chains of the nozzle pairs are arranged opposite of each other at a specified angle.
 5. Burner according to claim 2, wherein the angular position of the individual nozzles or the nozzle chains of the nozzle pairs is adjustable.
 6. Burner according to claim 1, wherein the other nozzle or the nozzle pair is provided outside the burner, at a distance relative to the burner tube closest to the substrate, in the space between the substrate surface and the burner.
 7. Burner according to claim 1, wherein the other nozzle or at least one of the nozzle chains is provided, which create a tangential gas stream component relative to the tube configuration of the burner.
 8. Burner according to claim 1, wherein the other nozzle, the nozzle pair or the respective nozzle chain is mobile and adjustable.
 9. Burner according to claim 1, wherein the nozzle running in the longitudinal axis of the burner comprises two interlocked tubes with thick walls.
 10. Burner according to claim 9, wherein the tubes are positioned inside each other concentrically or deviating from the concentric arrangement, wherein the wall thickness of the tubes ranges between ≧2 mm to essentially 20 mM.
 11. Burner according to claim 9, wherein the inside of the tube assembly has a greater distance from the plasma space than the outer tube of the tube assembly.
 12. Burner according to claim 1, wherein the at least one precursor gas feed comprises a chamber for mixing and/or calming the gas stream.
 13. Burner according to claim 1, wherein the nozzle chains comprise a plate shape and a plurality of nozzle bore holes.
 14. Burner according to claim 1, wherein the burner tube assembly is surrounded by a protective gas, in particular a nitrogen curtain.
 15. Burner according to claim 1, wherein the nozzle running in the longitudinal axis of the burner is designed as a nozzle group for partial streams converging in the plasma.
 16. Burner according to claim 15, wherein the nozzle group comprises individual nozzles with different or modifiable nozzle cross sections or cross-sectional areas.
 17. Burner according to claim 1, wherein the other nozzle or the individual nozzles of the nozzle chain are designed as a slotted nozzle.
 18. Burner according to claim 1, wherein the other nozzle or the individual nozzles of the nozzle chain are designed as concentric nozzles with jet-forming properties.
 19. Burner according to claim 4, wherein the nozzle chain is designed as a multi-line nozzle chain with nozzles arranged on top of each other or offset opposite from each other.
 20. Burner according to claim 7, wherein the tangential gas component runs in the rotational direction of the plasma helix.
 21. Burner according to claim 8, wherein the other nozzle, the nozzle pair or the respective nozzle chain is adjustable toward the direction of the substrate motion.
 22. Burner according to claim 12, wherein the chamber is designed as a prechamber at the end of the burner away from the substrate, running in the longitudinal direction of the burner with the gas feed arranged on the side.
 23. (canceled) 